The present invention relates to an alkaline storage battery using a negative electrode including, as a main constituent material, a hydrogen absorbing alloy capable of electrochemically absorbing/desorbing hydrogen and, more particularly, relates to an improvement in this type hydrogen absorbing alloy.
Recently such a hydrogen absorbing alloy capable of electrochemically absorbing/desorbing hydrogen serving as an active material in a battery has attracted attention as a negative electrode material having high energy density. Investigation has been performed as to development of high-capacity storage batteries such as nickel-hydrogen storage battery or manganese dioxide-hydrogen storage battery using a combination of negative electrode of the type described above and available positive electrodes such as nickel positive electrode or manganese dioxide positive electrode. Conventionally, alloys having a CaCu.sub.5 type crystal structure and represented by the general formula AB.sub.m C.sub.n have been proposed as hydrogen absorbing alloys to be used for negative electrode of this type nickel-hydrogen storage battery (Japanese Patent Unexamined Publication No. 60-89066 and U.S. Pat. No. 4,487,817). In the above formula, A is selected one from the group consisting of misch metal, Y, Ti, Hf, Zr, Ca, Th, La and other rare-earth elements; B is at least one element selected from the group consisting of Ni, Co, Fe and Mn; m is an atomic ratio within the range made up as follows, m.ltoreq.3.5 for Ni, m.ltoreq.3.5 for Co, m.ltoreq.3.5 for Cu, m.ltoreq.2.0 for Fe, and m.ltoreq.1.0 for Mn; C is at least one element selected from the group of Al, Cr and Si; and n is an atomic ratio within the range made up as follows, 0.05.ltoreq.n.ltoreq.0.6 for Al, 0.05.ltoreq.n.ltoreq.0.5 for Cr, and 0.05.ltoreq.n.ltoreq.0.6 for Si. As shown in the aforementioned proposal, a part of an alloy, for example, an LaNi.sub.5 alloy, is substituted by other elements to prepare a multicomponent alloy so as to prevent the alloy from becoming fine power by repetition of charging/discharging cycles and prevent the generated alloy powder from being oxidized to thereby prolong the cycle life of the battery.
Further, in Japanese Patent Unexamined Publication No. 61-233969, a multicomponent alloy represented by the general formula LnNi.sub.x Mn.sub.y M.sub.z is proposed, in which formula Ln is a selected one of or a mixture of rare-earth elements; M is at least one element selected from the group consisting of Co, Cu, Fe, Al, Cr, Zn, Ti, Zr, Mo, Si and Mg; z is an atomic ratio within the range made up as follows, 0.ltoreq.z.ltoreq.0.2 for Co, 0.ltoreq.z.ltoreq.2 for Cu, 0.ltoreq.z.ltoreq.2 for Fe, 0.ltoreq.z.ltoreq.0.9 for Al, 0.ltoreq.z.ltoreq.1 for Cr, 0.ltoreq.z.ltoreq.0.5 for Zn, 0.ltoreq.z.ltoreq.0.3 for Ti, 0.ltoreq.z.ltoreq.0.3 for Zr, 0.ltoreq.z.ltoreq.0.3 for Mo, 0.ltoreq.z.ltoreq.0.5 for Si, and 0.ltoreq.z.ltoreq.0.3 for Mg; x is an atomic ratio of not smaller than 3.5; and y is an atomic ratio of not larger than 1.5. The purpose of such a multicomponent alloy is also to prevent the alloy from becoming fine powder due to repetition of charging/discharging cycles, that is, due to repetition of absorption/desorption of hydrogen, to thereby prolong the cycle life of the battery.
In such a prior art configuration, although the problem with respect to the deterioration due to repetition of charging/discharging cycles could be solved, a new problem has been caused in that the internal gas pressure of the battery is so increased in a rapid charging period that it becomes difficult to perform the rapid charging.
In general, in a nickel-hydrogen storage bettery, the principle of the sealed cell design is the same as the oxygen extinguishing mechanism in a nickel-cadmium battery which has been proposed by Neumann, and the chargeable capacity of the negative electrode is established to be larger than that of the positive electrode. That is to say, the negative electrode is made so as not to be perfectly charged to have a notcharged portion remained even after the positive electrode has been perfectly charged, so that a hydrogen gas is prevented from being generated from the negative electrode is an overcharging period and at the same time, an oxygen gas generated from the positive electrode is absorbed to the negative electrode by the reaction represented by the following formula (1) to keep the sealing state of the battery, EQU MH.sub.x +O.sub.2 .fwdarw.MH.sub.x-4 +2H.sub.2 O (1)
in which M is a hydrogen absorbing alloy.
In the case where the negative electrode constituted by a hydrogen absorbing alloy is charged electrochemically (to absorb hydrogen) according to the formula (2), however, the hydrogen generation reaction represented by the formula (3) is carried out competitively in the last stage of the charging period. ##STR1## in which M is a hydrogen absorbing alloy. EQU H.sub.2 O+e.sup.- .fwdarw.OH.sup.- +1/2H.sub.2 .uparw. (3)
The hydrogen generation reaction, which is a competitive reaction, is carried out in an earlier stage of the negative electrode charging period, as the negative electrode is charged more rapidly. Accordingly, even through the sealed nickel-hydrogen battery is constructed so that the chargeable capacity of the negative electrode is established to be larger than that of the positive electrode, a hydrogen gas is generated from the negative electrode in the rapid charging period and cannot be removed, thereby the internal gas pressure of the battery is increased remarkably. As the result, a gas as well as an alkaline electrolytic solution are leaked from a safety vent (in general, which is operated by pressure of 10 to 15 kg/cm.sup.2). There arises therefore a new problem in that deterioration of cycle life and lowering of safety are caused by the leakage of the electrolytic solution.